FIG. 7 shows a conventional brushless motor which includes stator 110, rotor 120 and Hall elements 130 working as a magnetism sensing device. Stator 110 includes multiple coils 115 wound on teeth 113 of a stator iron core. Each one of coils 115 is coupled together, thereby forming a three-phase stator winding. Rotor 120 includes rotor iron-core 121 and multiple rotor-magnets 123. Rotor 120 is held to be rotatable inside stator 110.
Hall element 130 is placed under rotor-magnet 123 substantially at an identical angular position to a centerline of teeth 113. Hall element 130 senses a magnetic flux leaking from rotor-magnet 123, thereby sensing a position of the rotor.
FIG. 8 shows an array of rotor-magnets 123 and its leakage flux working on Hall elements 130. The reference marks “Bop” and “Brp” shown in FIG. 8 indicate threshold values for Hall elements 130 to sense the leakage flux, and widths “a” and “b” show dead bands existing when Hall elements 130 senses the leakage flux. A greater dead band causes Hall elements 130 to sense the rotor position not so accurately, and incurs vibration and noise of the motor. A smaller dead band is thus desirable for the magnetism sensing device of brushless motors.
Related art of the magnetism sensing device of brushless motors is disclosed in patent document 1.
Patent Document 1: Unexamined Japanese Patent Application Publication No. 2006-14521.